This invention related to optical sensor devices, such as charge coupled devices (CCDs), and in particular to a method of making an optical sensor device with one or more metallization layer(s) capable of withstanding high temperature treatment to improve quantum efficiency and reduce dark current.
As is well known in the art, charge coupled devices are used in a variety of optical applications and are the active component in video cameras. Such devices consist of an array of overlapping field effect transistors with a polysilicon contact layer formed over the gate, and over which is placed a pyroglass protective layer with contact openings so that an overlying metalization layer forming interconnects can make contact with the exposed polysilicon.
Typically, an aluminum alloy is used as the metalization layer in semiconductor devices. The problem with aluminum is that it will not withstand temperatures in excess of about 450xc2x0 C., whereas in order to reduce dark current, CCDs need to be annealed at temperatures considerably higher than this. In the prior art, it is known to replace aluminum by tungsten, but the problem with tungsten is that it forms under stress, which causes it to flake, and also it does not adhere well to N+ and P+ silicon contacts.
An object of the invention is to alleviate the afore-mentioned problems in the prior art.
According to the present invention there is provided a method of making an optical sensor device subject to a high temperature anneal, comprising the steps of providing a structure including transistors with active areas including a polysilicon gate contact layer; and forming an interconnect layer on said active areas by: (i) depositing a first titanium nitride sublayer on said active areas including said polysilicon gate contact layer; (ii) depositing on said first titanium nitride sublayer a plurality of alternating titanium and titanium nitride sublayers in a collimated sputtering chamber alternately in the absence and presence of nitrogen to Form a composite sandwich structure such that compressive and tensile stresses in said alternating titanium and titanium nitride sublayers substantially cancel out; and wherein said titanium nitride sublayers are deposited as stoichiometric layers.
The titanium nitride/titanium (TiN/Ti) sandwich structure is capable of withstanding the anneal temperatures required to reduce dark current. The titanium is in tensile stress and the TiN in compressive stress. By suitably depositing the layers, the stresses cancel out and thus eliminate the flaking problems experienced in the prior art.
The top layer of the sandwich structure is also preferably TiN since titanium oxidizes quickly, but it could be titanium.
The structure is also simple to form since it can be applied in a single sputtering chamber. To switch from Ti to TiN, it is merely necessary to turn on the supply of nitrogen.
The Ti/TiN is preferably deposited in a common collimated sputtering chamber. The collimator is located below the titanium target and reduces the titanium flux on the lower side of the collimator so that stochiometric TiN can be deposited on the wafer. With the collimator TiN having a bulk resistivity of 50 micro ohm cm. can be obtained.
The invention also provides an optical sensor device including an interconnect layer to contact active areas, wherein said interconnect layer comprises a first layer of titanium nitride layer on said active areas; and a plurality of alternating titanium and titanium nitride layers to form a composite sandwich structure.